scan strategies and basic products (version 1.1) msc radar course paul ford lead instructor moip...
Post on 22-Dec-2015
219 Views
Preview:
TRANSCRIPT
Scan Strategies and Basic Products (Version 1.1)
MSC Radar Course
Paul FordLead Instructor MOIP Dartmouth
Acknowledgements
• Dave Ball and Tim Bullock for suggesting I participate
• Norman Donaldson (King Radar) for explanations and encouragement
• Paul Joe, Dave Hudak, Rob Nissen, Mike Leduc, Phil Chadwick, Mark Pilon, Steve Knott and many others over the years for discussions on radar and mesoscale meteorology
• The dozens of MSC interns and other practicing meteorologists I have worked with for their “darn good questions”
Outline
• EC MSC and NWS NEXRAD system specifications
• MSC and NEXRAD scan strategies
• Product Examples and Usage– Discussion and Personal Observations
• Summary
• Questions
Basic Specifications of MSC Radars
• Wavelength: 5 cm• Antennae: Parabolic 3.7 or 6.1m dishes• Transmitter type: Magnetron (random phase)• Peak Power: 250 kW• Pulse Lengths: 0.8 – 2 microseconds• Pulse Repetition Frequencies: 250 – 1200 pulses per
second
MSC Radar Antenna – new 6.1m dish
MSC Antennae
• Beamwidth (deg) = 70 / (Antenna diameter)• The diameter of the reflector of the new systems is 6.1m,
compared to 3.7m for the retrofit systems• Thus, the angular beamwidth (to half power) of the new
systems is narrower than in the retrofit cases (0.65 deg vs 1.1 deg)
• This improves resolution and useful range – antenna gain is higher when beamwidth is small
IDENTIFIER NAME BEAMWIDTH ANTENNA
XGO GORE 0.65 deg 6.1m
XMB MARION BRIDGE 1.1 3.7
WTP HOLYROOD 1.1 3.7
XME MARBLE MOUNTAIN 0.65 6.1
XNC CHIPMAN 1.1 3.7
XAM VAL D’IRENE 0.65 6.1
WMB LAC CASTOR 1.1 3.7
Pulse Repetition Frequency (PRF)
• The pulse repetition frequency (PRF) is the number of pulses emitted by the radar per second (pps)
• A pulse travelling to a target at range rmax and back will cover a distance 2rmax
• The pulse will make it back to the radar before the next pulse is emitted if: 2rmax=c/PRF
PRF and radar range
• Thus, the higher the PRF, the lower the effective range (ignoring second-trip echos from objects located beyond rmax)
• Recall, higher PRFs are necessary in Doppler mode to derive a greater range of velocities (Doppler dilemma)
• Dual PRF technique used in MSC system to extend Vr range to 48 m/s
rmax=c/2PRF
Rmax in selected modes
Mode PRF Rmax (km)
Reflectivity 250 600
Doppler 900 &1200
125
Clear Air 50 3000
Rmax vs PRF
0
100
200
300
400
500
600
700
PRF
R m
ax
(k
m)
NEXRAD Transmitter Specs
• Type: S-band (10 cm), coherent chain (STALO/COHO), line modulator, klystron tube amplifier (53 dB gain typical)
• Frequency: 2700 to 3000 MHz • Power: 750 kw peak at klystron output • Average Power: 300 to 1300 watts • Pulse Widths: 1.57 and 4.5 microseconds (-6 dB points) • PRF short pulse: 318 to 1304 Hz • PRF long pulse: 318 to 452 Hz
NEXRAD Antenna Specs
• Type: center fed paraboloid of revolution 28 feet (8.5m) in diameter • Polarization: linear horizontal • Gain at 2850 MHz: 45.5 dB (including radome loss) • Beamwidth at 2850 MHz: 0.925 deg • First sidelobe: -29 dB (others less than -40 dB beyond 10 deg) • Radome: fiberglass foam sandwich frequency tuned, 39 foot
truncated sphere
NEXRAD Reflectivity Product
• Reflectivity computation: linear average return power • Reflectivity estimate standard deviation: less than 1 dB typical • Number of pulses averaged: 6 to 64 • Range increment: 1000 m • Max range for reflectivity: 460 km • Signal Detection Capabilities (at 0 dB SNR)
– Minimum required signal detection, short pulse -7.5dBZe at 50 km– Typical Detection (for Ze=200*R1.6) -10 dBZe at 50 km (rainfall of 0.01mm/hr)– Minimum required signal detection, long pulse -23.0dBZe at 25 km
NEXRAD Velocity Base Product• velocity computation: complex covariance argument (pulse pair
estimator)
• velocity estimate standard deviation: less than 1 m/sec (at spectrum width of 4 m/sec)
• number of pulses averaged: 40 to 200
• range increment: 250 m
• azimuth increment: 1 deg
• max range for velocity: 230 km
Volume Scans• Definition: a series of consecutive radar scans that together sweep out a
volume of the atmosphere• MSC radars perform separate conventional and Doppler volume scans• Some data processing is done by a computer at the radar site• Image products are produced by MSC’s Unified Radar Processor (URP)
which is run on computer servers in each of the storm prediction centres (SPC) across Canada
• Users may access any of these servers remotely and request images using the Interactive Viewer or the NinJo workstation
MSC Radar’s Volume Scans
• Nominal times of the scans are 0, 10, 20, 30, 40 and 50 minutes of each hour
• The conventional scan (CONVOL) is completed in the 5 minutes before the nominal time
• 4 Doppler scans (DOPVOL) are performed in the 5 minutes after the nominal time
Basic MSC Antenna “Scan Strategies”
• Conventional (CONVOL): – 24 PPI scans– top down– 24.6 to 0.3 degrees– 6 RPM– 5 minutes to complete
• Doppler (DOPVOL1a,1b,1c):
– 3 scan angles (LOLAA, 1.5 and 3.5 degrees)
– much slower (0.85 RPM) since more sample points are collected for the Doppler processing
– fills next 4.5 minutes
• Doppler (DOPVOL 2):– BALD angle
– ‘rapid’ scan, made during the last 30 seconds of the Doppler cycle, noisy (low S/N)
– fills last 30 seconds (~2 RPM)
Implications of the MSC CONVOL
• 1.5 km CAPPI becomes a PPI beyond about 130 km– Overshooting of distant shallow precipitation
• Tops of tall (e.g. cumulonimbus) cells within 20-30 km of the radar are often underestimated
– This can lead to a misdiagnosis of the extent of growth or decay of cells close to the radar
U.S. NEXRAD Scan Strategies – VCP is the “Volume Coverage Pattern”
VCP Scan Time (min) Elevation angles (°) Usage Special attributes
11 5
0.5, 1.5, 2.4, 3.4, 4.3, 5.3, 6.2, 7.5, 8.7, 10, 12, 14, 16.7, 19.5
Convection, especially when close to the radar
Has the best overall volume coverage.
12 4
0.5, 0.9, 1.3, 1.8, 2.4, 3.1, 4.0, 5.1, 6.4, 8.0, 10.0, 12.5, 15.6, 19.5
Convection, especially activity at longer ranges
Focuses on lower elevations to better sample the lower levels of storms.
121 5.50.5, 1.5, 2.4, 3.4, 4.3, 6.0, 9.9, 14.6, 19.5
Large number of rotating storms, tropical systems, or when better velocity data is needed.
Scans lower cuts multiple times with varying pulse repetitions to greatly enhance velocity data.
21 60.5, 1.5, 2.4, 3.4, 4.3, 6.0, 9.9, 14.6, 19.5 Shallow precipitation
Rarely used for convection due to sparse elevation data and long completion time.
31 10 0.5, 1.5, 2.5, 3.5, 4.5
Detecting subtle boundaries or wintry precipitation Long-pulse
32 10 0.5, 1.5, 2.5, 3.5, 4.5
Slow rotation speed allows for increased sensitivity. Default clear-air mode, reduces wear on antenna. Short-pulse
NEXRAD VCP11-Convection Mode
NEXRAD VCP21 – Shallow Precip Mode
NEXRAD VCP31-Clear Air Mode
Let’s go to MSC’s Unified Radar Processor to look at some examples of products from MSC and US radars!
MSC Conventional Products Derived from CONVOL
Composite Products
• Composed of data of similar type– PRECIP product or NEXRAD base reflectivity– CAPPI 1.0 km– CAPPI 1.5 km– ECHOTOP– 3 hr Precipitation Accumulation
• MSC URP default for composites is “maximum value”
MSC Doppler Products Derived from the DOPLVOL scans:
(DOPVOL1a,b,c and DOPVOL2)
MSC Fast Fourier Transform (FFT) Filtering of artifacts - Doppler products only
Construction of the Velocity Azimuth Display (VAD)
Shallow snow – moisture only up to ~2 km on upstream sounding
NEXRAD Products
• A lot of similarity to MSC products
• All based on PPI’s, no CAPPI’s
Base Reflectivity (Labelled CLOGZ1 in MSC URP)
NEXRAD ‘Composite’ Reflectivity (COMPZ MAXR)
NEXRAD Echo Tops
NEXRAD Base Radial Velocity: Labelled VR 1 in MSC URP
Doppler Product Usage: Bright Band Detection
Doppler Product Usage: Mesoscale Boundaries and
Circulations
Example: Eastern Ontario Convection Case
• generation of gust fronts/outflow boundaries
• mesocyclone signatures
• interaction of the mesoscale boundaries with roll clouds– convective initiation
Franktown Radar
DivergenceSignature
MesocycloneSignature
Thunderstorm Outflow Boundaries
Another Gust Front
Lake Breeze Fronts
Lake Breeze Fronts
Summary• Conventional data is used in:
– Weather surveillance
▪ CAPPIs, composites– Cross sections, severe cell identification and assessment
– Multi-level products (MAXR, ECHOTOP)
• No clutter suppression
• Coarser native resolution (sampling only every degree and km)
• Higher minimum displayed value – weaker returns not displayed
Summary
• Reflectivity and radial velocity data displayed
• Doppler products are all PPI’s– Can derive profiles of reflectivity and radial velocity
• Higher resolution and weaker returns (0.5 degree and 0.5 km sampling) from slower antenna rotation rate and better sampling
– Mesoscale boundary detection possible
• BALD (Precip and LR products) and LOLAA (lowest CLOGZ PPI) is capable of seeing shallow precipitation farther out
Thank You!
Questions??
top related